Renix High Idle at Startup

Ive gone thru hours reviewing the topic of Renix High Idle at Startup and havent found anything that seems to fit the problem on my 88 XJ.

As the subject line states when I start up my XJ it revs up high (no tach to watch) for 8-10 seconds. Then it settles back to whatever the idle speed is. Other than that everything is fine. Its done this for years I think. Ive had the IAC out and it lookks fine. DOnt know if it moves or not when the XJ is running.

Today after I got to work I removed the electric plug going to the IAC. After work I started the XJ up and it started right up WITHOUT the High Idle at Startup. The Jeep ran just fine all the eay home with a couple of stops/starts on the way.

Ideas? Im wondering if it could be the "latch relay".

Thanks.

Try cleaning out the TPS (throttle position sensor on the throttle body) and C101 (on the driverside cowl underhood my the emissions label used on 87-88 models) connectors first and see if that helps. It could be the TPS or adjustment, wiring, connections, or even ECM.

The B+ latch relay is controlled by the ECM. It keeps the relay energized after the engine is shut down for a little time so the idle motor can be adjusted for the next startup, possible I suppose.

Ok, think about this: the IAC is a simple extend/retract stepper motor that responds to the ECU/PCM's commands. So, the IAC is probably just doing what it is told to. Now, as far as an ECU/PCM goes, well they are Ok, but about as dumb as a box of rocks. They are issuing commands to the IAC based on what sensors tell it. And sensors are notorious liars.

After all that BS, I would first check the EGR system to make sure it isn't screwing up. After that I would suspect the TPS is bad or out of adjustment. You can use a digital multimeter to test the grounds and input voltage, as well as adjusting the TPS to give the correct output voltage. However, and this is a biggee, you cannot use a DMM to test the TPS throughout its range of motion, as MOST DMMs have buffering circuits that will hide TPS flat spots. I have been told that Fluke meters can handle this job.

Once you have the tested and properly adjusted (or replaced and properly adjusted) the TPS, if the problem persists you will have to check the ECU/PCM pin outs and harness for any issues.

you are also going to have to get your hands on a tach. You need to determine if the idle speed is correct. Idle, controlled by the ECU/PCM through the IAC, should be 700~750 rpm. Manual is checked in neutral, engine fully warmed up. Auto is checked in "D" with both the engine and trans fully warmed up--after about 20 minutes of operation.

Thanks fellows. I'll check it out.

Just wondering, how high do you think your idle is going up when you first start up.

The IAC is entirely controlled by the ECM, based primarily by engine temp at start-up.
At the previous shut-down, the ECM forces the IAC open, in anticipation of the next start-up. It's correct for the engine in a RENIX system, to flare up at start-up. The question is how much and for how long. I know longer have the 4.0L Renix in my Jeep, but as I recall, 1200, or so RPM at start-up is correct. The RPMs then should stay high until the engine warms up. But a re-start on a warm engine should still experience high idle, but for a very short time. The ECM knows the engine temp, and has an idle RPM in mind, and should adjust the start-up idle RPM quickly. Without a tach connected to your engine, it would be hard to say you have a problem.

Gonna revive an old thread here, did you guys ever figure out the problem. My renix is doing this exact same thing, warm starts, rpm up to about 2200 for 3 to 5 seconds then settles down to the normal 700.

I've check all the sensors as per the renix manual and everyone checks out and is giving back the correct ohms value or voltage return.

I'm stumped? Anyone have an idea?

The most common things that cause this are a bad TPS, wiring connections (especially the C101 connector on 87/88 models), and the ECM itself. If the voltage signal from the TPS changes after key on to starting it will cause the ECM to high idle the engine.

Like the thread said, clean the IAC pintle and throttle body where it sits, and replace the relay.

If the TPS measures fine then it is definitely not the TPS. You might also want to check for a vacuum leak.

old_man said:

If the TPS measures fine then it is definitely not the TPS. You might also want to check for a vacuum leak.

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Not if the readings jump around as the throttle is touched or opened even it they are the right voltages when adjusting. Erratic readings will also cause the problem.

Mine used to do that and I wish I could tell you exactly what fixed it. One thing that really had an impact was my TPS ground, it was sitting in an uncrimped splice and just kind of making contact. I never really could figure out why my TPS adjustment was different most ever time I checked it, I never could get it to repeat to exactly the same numbers twice. I got lucky one day and shook the harness up near the firewall while I was doing a TPS ground ohm check and watched the needle on my analog meter jump around. I ran the TPS wires over the top of the rubber TB intake and stripped back a little insulation on the wires, which made testing a whole lot easier. I eventually left the wires up there. One thing that kind of stunned me was when I figured out the ground wire for the ECU half of my TPS ran through the TCU and the ground for the TCU half of my TPS ran through the ECU ground circuit, or in other words bass ackwards. I'm sure they aren't all like that, but mine was.

For me it seemed to only happen (bad) on a hot motor. Tightening my manifold bolts and curing some minor vacuum issues seemed to help. I clean and lube my IAC on a regular basis so dirt or sludge wasn't likely an issue.

I was having some charging issues which was likely also a factor, my IAC (I tired three different IAC's) was kind of sluggish when I was having charging issues. Sometimes it would stick shut, so it was likely it sometimes also stuck open.

The way they did splices on these was pressing the wires together to kind of fuse them and wrapped in a silver duct tape. Best to solder these together. Poor connections lead to erratic voltage. I understand that a lot of folks when encountering a grounding issue simply tie it to an available ground which works good enough I guess in most cases but the factory runs an isolated ground to the sensors so what effects one effects them all the same and is doesn't effect operation. If you have a different ground then it will effect that sensor different which the ECM might see as a problem.

Click on the link below and do posts 1 through 5.

http://www.cherokeeforum.com/f51/cruiser54s-mostly-renix-tips-153657/

If it persists, check the TPS adjustment, Post 8.

Actually, I advocate post 27, eliminating the C101 altogether.

cruiser54 said:

Actually, I advocate post 27, eliminating the C101 altogether.

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Did that on my '88. Back in the day we used to replace the 87-88 wiring with the updated 89-90 ones.